|Date||10th of January 2012|
|Credit||Ray Morris of BetterCGI.com.|
|Affects||Most PowerDNS Authoritative Server versions < 3.0.1 (with the exception of the just released 18.104.22.168)|
|Not affected||No versions of the PowerDNS Recursor (‘pdns_recursor’) are affected.|
|Impact||Using well crafted UDP packets, one or more PowerDNS servers could be made to enter a tight packet loop, causing temporary denial of service|
|Exploit||Proof of concept|
|Risk of system compromise||No|
|Solution||Upgrade to PowerDNS Authoritative Server 22.214.171.124 or 3.0.1|
|Workaround||Several, the easiest is setting: cache-ttl=0, which does have a performance impact. Please see below.|
Affected versions of the PowerDNS Authoritative Server can be made to respond to DNS responses, thus enabling an attacker to setup a packet loop between two PowerDNS servers, perpetually answering each other’s answers. In some scenarios, a server could also be made to talk to itself, achieving the same effect.
If enough bouncing traffic is generated, this will overwhelm the server or network and disrupt service.
As a workaround, if upgrading to a non-affected version is not possible, several options are available. The issue is caused by the packet-cache, which can be disabled by setting ‘cache-ttl=0’, although this does incur a performance penalty. This can be partially addressed by raising the query-cache-ttl to a (far) higher value.
Alternatively, on Linux systems with a working iptables setup, ‘responses’ sent to the PowerDNS Authoritative Server ‘question’ address can be blocked by issuing:
iptables -I INPUT -p udp --dst $AUTHIP --dport 53 \! -f -m u32 --u32 "0>>22&0x3C@8>>15&0x01=1" -j DROP
If this command is used on a router or firewall, substitute FORWARD for INPUT.
To solve this issue, we recommend upgrading to the latest packages available for your system. Tarballs and new static builds (32/64bit, RPM/DEB) of 126.96.36.199 and 3.0.1 have been uploaded to our download site. Kees Monshouwer has provided updated CentOS/RHEL packages in his repository. Debian, Fedora and SuSE should have packages available shortly after this announcement.
For those running custom PowerDNS versions, just applying this patch may be easier:
--- pdns/common_startup.cc (revision 2326)
+++ pdns/common_startup.cc (working copy)
@@ -253,7 +253,9 @@
It should apply cleanly to 3.0 and with little trouble to several older releases, including 2.9.22 and 2.9.21.
This bug resurfaced because over time, the check for ‘not responding to responses’ moved to the wrong place, allowing certain responses to be processed anyhow.
We would like to thank Ray Morris of BetterCGI.com for bringing this issue to our attention and Aki Tuomi for helping us reproduce the problem.